Condensation on stone floor
Southwestern Massachusetts early 1980’s construction. I can’t figure out exactly why my client’s entry floor seems to have condensation. Wall Construction is: vertical wood siding, felt wrb, 1/2 cdx sheathing, 2×6 framing with fiberglass batts, 1/2 drywall (no interior poly). Floor construction: 1/2 cdx subfloor, 3/4” bluestone embedded in mortar. Only about half of the small entry floor is showing condensation, not sure why. I replaced the entry door last winter. I noticed damaged drywall that seemed to be wicking moisture from the floor- the old drywall had been installed right to the stone. I could not find any evidence of exterior water infiltration, so I guessed that if my new (mold resistant) drywall was left 1” off the stone the moisture wicking would not continue. Apparently I was wrong as through this hot and humid weather my client cleaned “little black spots” off the surface of the new wall. I told her I can’t understand how the wall can wick the moisture, and don’t feel I have a clear enough understanding of the problem to implement a solution. Anyone have suggestions, or can you recommend the right person the visit the site for proper diagnosis?
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Replies
Forgot to mention there is a full basement under this stone floor, with fiberglass batts only at the perimeter and open joists...
Condensation is a two-part issue: 1. Something is chilling the surface. 2. Somehow, warm/wet air is getting to the surface.
I would look for air from an A/C vent chilling the surface. There might also be a leaking A/C duct in the basement under the surface.
The warm/wet air might be coming through the front door. Even with a new entry door, most of them leak air under the threshold sweep. It's pretty hard to make that seal airtight. Turn on all exhaust fan equipment and test it with a smoke pencil.
Lukas,
Peter gave you a good answer. You have to figure out why the stone floor is cold, and you have to figure out why the interior air near the stone floor is humid.
You description of the wall assembly details and the rim joist insulation didn't mention air barriers or airtightness, so there are lots of issues that may need to be remedied. To focus on your main problem, however, you need to understand the temperature of the flooring -- an infrared thermometer might help -- and determine why the flooring is cold. Then you have to check for a source of humid air -- either (a) air leaks that allow outdoor air to come in contact with the cold flooring (if the condensation happens during the summer), or (b) interior air that is too humid.